In the manufacture of flat rolled metal it is most convenient and economical to form the web of a much greater width than is normally required by the end user and then slit the web into narrower strips of a suitable width. The metal web is coiled as it is processed, then, in a separate operation, placed on an uncoiler, unwound, trained through a slitter and then rewound as a number of separate narrower strips on the coiler. The slitting operation may be accomplished at the point of manufacture, by middlemen, such as warehousemen, or by the end user of the sheet metal.
Regardless of at what point the coil slitting takes place, inherent characteristics of the sheet metal and conventional coil slitting processes result in a number of difficulties to which the industry has responded in a manner which, in many cases, only solves the problems encountered by producing other, different problems.
For example, although the sheet of metal being slit is generally thought of as having a rectangular cross-sectional configuration, in fact, conventional sheet metal manufacturing processes produce a sheet which is crowned, i.e. is thicker, at its center than at its edges. Obviously, as such a sheet is rewound on a coiler as a series of separate strips following slitting, those strips slit from the center of the sheet are thicker and as a result are rewound more tightly than those strips slit from adjacent the edges of the sheet. This in turn results in so called "slack strands" being formed by the thinner strips between the slitter and coiler. To overcome the problem of slack strands a number of solutions have been advanced, and in fact are found in use today throughout most coil slitting operations.
One approach has been to insert pieces of cardboard or paper between the wraps of those coils positioned outwardly of the center coil to compensate for the differences in thickness of the strips being rewound. This is often performed manually, which is both cumbersome and dangerous, and even where performed mechanically is still cumbersome and requires a specially designed machine. In both cases, the cardboard or paper pieces must be removed later as the strip is decoiled for punching, pressing or other operations.
Two other, related approaches to the problem of slack strands are the looping and festooning of the strands intermediate the slitter and the coiler. Looping requires the provision of a deep pit, which is both inconvenient and expensive, while festooning requires the installation of a series of rolls mounted in towers above the process line, an obviously costly expedient, and in both looping and festooning control of the slack stands is always a problem.
While individual coilers could be provided for each of the strips resulting from the coil slitting operation, as a practical matter the expense of such provision will usually be prohibitive. Another approach which is based upon individual treatment of the slit strips but which does not require separate coilers is slip core winding. In this process, the strips are wound on nonmetallic cores that are allowed through friction to wind at a speed commensurate with the thickness of the strips. However, the cores used in this operation are in themselves expensive and must be retained within and shipped with the coils, and in addition they may distort under load and cause irregular winding.
Another problem characteristic of conventional coil slitting operations which is independent of the crowned configuration of the metal sheet and would, therefore, exist even if the sheet were perfectly rectangular in cross section, is interleaving of the strip edges as they are rewound on the coiler. Interleaving in turn results in damage to the edges of coil, loss of production time resulting from the necessity of manually separating interleaved coils and difficulties in feeding such coils, because of their damaged edges, through machinery such as punching presses and the like.
To prevent interleaving during rewinding, an attempt is generally made to keep the individual strips separate from each other. This may be accomplished by positioning spacer plates between coils or through the use of a series of discs which are mounted on a shaft separate from the coiler and allowed to penetrate between the coil edges as they are rewound.
Regardless of the particular manner in which separation is attained, it will be seen that separation requires lateral displacement of the individual strips from each other. This in turn requires that the coiler be spaced a considerable distance from the separator to allow the strips to fan out gradually from the slitter to the required spacing at the coiler. Ordinarily, to obtain a total lateral displacement of approximately two to three inches it is necessary to provide from fifteen to twenty feet of spacing between the slitter and the coiler.
From the above it will be apparent that conventional coil slitting operations possess many inherent disadvantages and present many problems which have traditionally either been accepted or only partially solved, often at the expense of introducing other difficulties and new problems into the process. A need therefore, has long existed for a new approach to coil slitting which obviates the problems of slack strands and coil interleaving and all of their attendant disadvantages.